1. Field of the Invention
The invention relates to an AC backup power system and, in particular, to a power system that ensures that the AC power is supplied to a power supply at the zero-crossing point to avoid problems such as sparks and coke deposition.
2. Description of Related Art
To ensure the stability of power supply, advanced power supply systems all have a redundant design. The so-called redundant power system has two or more power modules working together. When one power module fails and cannot supply power normally, the other power module can immediately take over the job of the failed power module. The above-mentioned redundant design is generally for the load. For input power of the power supply system, there is usually only a single AC power source. Therefore, once the AC power source fails, no power can be supplied to the load even though the power system has a redundant design.
In order to solve the above problems, an AC power supply with the AC backup function has been proposed, as shown in FIG. 5. The AC power supply includes first, second, and third DC power supply modules 71, 72, 73 and a switching circuit 70.
Output terminals of the first, second, and third DC power supply modules 71, 72, 73 connect in parallel to supply power to the load. Input terminals of the first and second DC power supply modules 71, 72 connect respectively to a first and a second AC power source AC1, AC2. The switching circuit 70 has first, second, and third relays 701, 702, 703. The third relay 703 is a 2-to-1 type, having two input terminals and one output terminal. The two input terminals of the third relay 703 connect to the first and second relays 701, 702, respectively. The output terminal of the third relay 703 connects to the input terminal of the third DC power supply module 73. The input terminals of the first and second relays 701, 702 connect respectively to the first and second AC power sources AC1, AC2.
When the first and second AC power source AC1, AC2 operate normally, they provide power to the first and second DC power supply modules 71 and 72, respectively. The first AC power source AC1 also supplies power to the third DC power supply module 73 through the first and third relays 701, 703. Therefore, the first, second, and third DC power supply modules 71, 72, 73 output power in parallel.
If the first AC power source AC1 is interrupted, the first relay 701 opens and the second relay 702 turns from open to closed. The third relay 703 switches to connect to the second relay 702. In this case, the first DC power supply module 71 stops working because of the breakdown of the first AC power source AC1. The second DC power supply module 72 keeps working because the second AC power source AC2 is normal. The third DC power supply module 73 obtains power from the second AC power source AC2 via the third and second relays 703, 702, thereby working in parallel with the second DC power supply module 72.
The above-mentioned case specifically emphasizes that the switching of the switching circuit 70 must be done at a particular time, i.e. the zero-crossing point, of the AC power so as to avoid surges and sparks while switching In practice, a microprocessor is employed to monitor the AC power, in the hope that the microprocessor can precisely control the relays to switch exactly at the zero-crossing point. However, the above-mentioned case uses the relays as the switching elements, and the relays are mechanical switches. It has the problem of time delay. Even if the microprocessor detects the zero-crossing point, the actual response of switching operations made by the relays do not occur at the zero-crossing point. Since the switching operations are not at the zero-crossing point, there are generally surges and sparks, which still result in the coke deposition problem. So the problem in the prior art is not effectively solved.